As well known in this field, optical disks having multiple recording layers include a guide-layer integrated disk having a recording layer and a guide layer formed in the same layer for each recording layer and a guide-layer separated disk having recording layers and a guide layer formed separately from the recording layers. The guide layer is a layer in which a servo guide structure or signal, containing position (address) information, is formed as a guide track.
Regarding the guide-layer integrated disk, even non-recorded portions with no recording layer information recorded can be provided with tracking control using the guide track integrated with the recording layer, enabling information to be recorded on the track specified by the guide track (see Patent References 1 and 2). This disk also has an advantage of being capable of reading and writing information using a single laser beam.
On the other hand, the guide-layer separated disk requires a servo laser beam for reading on a guide track in the guide layer and a read/write laser beam for writing information or reading recorded information on the recording layer. To record information on one recording layer, while tracking control is provided to move the focal position of the servo laser beam along the guide track of the guide layer, the read/write laser beam is focused on one recording layer to write information (see Patent Reference 3). Accordingly, the optical disk drive apparatus includes a servo optical system for irradiating the guide layer with the servo laser beam. The apparatus also includes a read/write optical system for irradiating the recording layer with the read/write laser beam to detect the reflected beam using the same objective lens as that of the servo optical system to detect the reflected beam. This guide-layer separated disk is formed of simple structure recording layers stacked in layers, thus allowing optical disks to be manufactured with ease at reduced costs. The disk also has an advantage, over the guide-layer integrated disk, of having an increased recording capacity because multiple recording layers can be formed with ease.
To write information or read recorded information on an optical disk with multiple recording layers, a desired recording layer to be read or written has to be quickly accessed. That is, it has to be quickly determined for the guide-layer integrated disk whether the recording layer on which a single laser beam has been generally focused is the desired recording layer, and for the guide-layer separated disk whether the recording layer on which the read/write laser beam has been generally focused is the desired recording layer. Note that as used herein, the expression, “to access the recording layer” refers to a series of operations for focusing a laser beam on the desired recording layer by focus servo control.
Patent Reference 1 discloses that when preformats, including addresses or the like, of each recording layer of the optical disk are the same for each recording layer, the preformats are mutually shifted in the direction of rotation of the disk between each recording layer. Upon recording or reproducing, the amount of phase shift from a reference recording layer is detected to know which recording layer is being currently accessed.
Furthermore, Patent Reference 2 also discloses that part of each recording layer of the optical disk is provided with a reflective film coated portion having a high reflectivity that allows for focus servo retraction, and the reflective film coated portion of each recording layer is located at different positions.
Related Patent Applications
Patent Reference 1: Japanese Patent Application Publication No. 2002-334448
Patent Reference 2: Japanese Patent Application Publication No. 2005-122862
Patent Reference 3: Japanese Patent Application Publication No. Heisei 4-301226